Light shielding sheet

ABSTRACT

Provided is a light-shielding sheet comprising a light-shielding material made of a white film having a light transmission of not more than 50% and a reinforcement made of a textile fabric or a nonwoven fabric. Preferred is a case where the white film is a film having therein voids formed through stretching of the film or a film having therein bubbles formed through foaming by use of a foaming agent, whereby a light-shielding sheet is provided which has a light weight, a high strength and a superior durability and which inhibits temperature increase caused by the heat resulting from sunlight absorption. The light-shielding sheet is employed suitably as a light-shielding material for agricultural use and horticultural use.

TECHNICAL FIELD

[0001] The present invention relates to a light-shielding sheet, andmore particularly to a light-shielding sheet used in various fields suchas agriculture, stock raising, construction, tents, etc.

BACKGROUND ART

[0002] Films made of thermoplastic resin containing light-reflectingpigment typified by powdery aluminum and white pigment such as titaniumoxide or light-absorbing pigment such as carbon black added thereto haveheretofore been used as a light-shielding material which is employedmainly as a covering material for greenhouses in the fields ofagriculture and horticulture. Making a film containing alight-reflecting pigment such as powdery aluminum and white pigment havea light transmission reduced to a certain level or lower requiresaddition of a large amount of pigment. The addition of a large amount ofpigment will cause problems, e.g. reduction in productivity such asbreakage at stretching into a thin film and reduction in mechanicalproperties. Further, the films containing a light-absorbing pigment suchas carbon black are problematic in that the films absorb the light inthe daytime and release it as heat in the night to cause the temperatureincrease in houses.

[0003] A light-shielding material made of a thermoplastic resin filmsuch as that mentioned above is so poor in durability that when it isemployed alone as a covering material for a house, it is easily brokendue to the force applied to the film during the work of spreading it asa house covering material or during the daily-conducted opening andclosing work.

[0004] JP-A 11-333978 discloses a translucent heat-insulating sheetobtained by laminating an aluminum deposition film and a woven fabricmade of flat yarns of high density polyethylene by sandwich lamination.However, the aluminum deposition film is not suitable for folding or thelike during the opening and closing work as a house covering materialand is problematic in that the aluminum deposition layer peels off.

[0005] JP-A 9-172883 discloses a light-shielding net obtained by weavingflat yarns with tape-like fiber lines, which were used as warps andwefts, produced by slitting a nonwoven fabric composed of syntheticresin filaments piled and bonded in high density and in randomdirections. JP-A 10-327684 discloses a light-shielding net comprising anet base fabric obtained by weaving flat yarns, onto one side of the netbase fabric tape-like fiber lines produced by slitting a nonwoven fabriccomposed of synthetic resin filaments piled and bonded in high densityand in random directions have been welded while being spaced apart fromone another. However, films or tapes made of such light-reflectingnonwoven fabric are unsatisfactory in abrasion resistance and instrength. They are also defective in handlability at the time ofopening, closing and folding because they are bulky. Moreover, they arepolluted during their outside use by, for example, adhesion of dust orgeneration of moss caused by entrance of rain water, then their lightreflectance easily decrease. In addition, such nonwoven fabric is moreexpensive than normal films.

[0006] The present invention was made in light of the problems mentionedabove. The object of the present invention is to provide alight-shielding sheet which has a lightweight, a high strength and asuperior durability and which inhibits the temperature increase causedby absorption of heat from sunlight.

DISCLOSURE OF THE INVENTION

[0007] The object of the present invention is achieved by alight-shielding sheet comprising a light-shielding material made of awhite film having a, light transmission of not more than 50% and areinforcement made of a textile fabric or a nonwoven fabric.

[0008] It is preferable that the white film be a film having thereinvoids formed through stretching the film. In addition, it is alsopreferable that the white film be a film having therein bubbles formedthrough foaming by use of a foaming agent.

[0009] At least one of the light-shielding material and thereinforcement preferably contains a weathering agent, and morepreferably contains a weathering agent and an antioxidant. Moreover, itis preferable that a protective film be laminated on a surface of thelight-shielding material and that the protective film contain aweathering agent. It is more preferable that the protective film containa weathering agent and an antioxidant. It is also preferable that thereinforcement be a woven fabric obtained by weaving a drawn yarn made ofpolyolefin as warps and wefts.

[0010] A light-shielding sheet in which the reinforcement is laminatedon the back side of the white film is a preferred embodiment. Anotherpreferred embodiment is a light-shielding sheet in which tapes resultingfrom slitting the white film into a narrow width are laminated on oneside of the reinforcement while being spaced apart from one anotherapproximately in parallel. Still another preferred embodiment is alight-shielding sheet comprising a woven fabric obtained by weaving adrawn yarn with a flat yarn as warps and/or wefts, the flat yarnresulting from slitting the white film into a narrow width.

[0011] The present invention is described in detail below.

[0012] The white film in the present invention has a light transmissionof not more than 50%. The invention uses this white film as alight-shielding material. A white film having a light transmission ofmore than 50% demonstrates only an insufficient light-shieldingproperty. Use of such a white film makes it difficult to achieve theobject of the present invention.

[0013] There is no particular limitation with respect to the startingresin for the white film. Various types of thermoplastic resin such aspolyolefin, polyester, e.g. polyethylene terephthalate and polybutyleneterephthalate and polyamide, e.g. nylon 6 and nylon 6,6 are suitablyemployed. Of these resins, polyolefin is preferably used in view ofproductivity, drawability and void formability. Specific examples of thepolyolefin include polyethylene resins such as high densitypolyethylene, medium density polyethylene, linear low densitypolyethylene, branched low density polyethylene, ultra low densitypolyethylene, linear low density polyethylene produced by use of ametallocene catalyst, ethylene-vinyl acetate copolymer, ethylene-alkyl(meth)acrylate copolymer: and polypropylene resins such aspolypropylene, ethylene-propylene block copolymer and ethylene-propylenerandom copolymer. Above all, polypropylene is particularly preferredfrom the viewpoints of performance and cost.

[0014] Preferable examples of the white film are as follows. One exampleis a film having therein voids (vacancies by pores) formed throughstretching the film. Another example is a film having therein bubbles(cavities) formed through foaming by use of a foaming agent.

[0015] The method for obtaining a white film by forming voids in a filmby stretching the film is described first.

[0016] Voids can be formed by mixing void-forming particles to thestarting resin, fabricating the mixture into a film, and then stretchingthe film. The void-forming particles may be of either incompatible resinor inorganic filler. However, in the case of being incompatible resin,the particles must be incompatible with the starting resin at thestretching temperature. Among those available materials, incompatibleresin is preferred. The incompatible resin and the inorganic filler maybe used in combination.

[0017] When the polyolefin is employed as the starting resin, examplesof the incompatible resin include polyolefin such as polymethylpentene;polystyrene; polyester such as polybutylene terephthalate; nylon; andpolycarbonate. The incorporation amount of these incompatible resins iswithin the range of 3-30% by weight, and preferably within the range of3-10% by weight. If the incorporation amount is less than 3% by weight,only a small amount of voids will be formed and it will become difficultfor the condition that the light transmission is not more than 50% to besatisfied. On the other hand, an incorporation amount of over 30% byweight is undesirable because it may result in loss of the inherentproperties of polyolefin.

[0018] Examples of the inorganic filler include white pigment such astitanium oxide, zinc flower and zinc sulfide; silver pigment such asaluminum powder; and calcium carbonate. The particle size of theinorganic filler is preferably 0.1-1 μm. The incorporation amount of theinorganic filler is preferably 2-20% by weight. An incorporation amountof less than 2% by weight is undesirable because it will result information of a small amount of voids. On the other hand, anincorporation amount of over 20% by weight is also undesirable becauseit may cause falling off of an inorganic filler powder exposed at thesurface of a film during the stretching thereof.

[0019] Examples of the method for stretching a film include methods ofuniaxially or biaxially stretching an unstretched sheet to apredetermined drawn ratio by a known technique such as rolling andtentering. Preferred is biaxial stretching. This stretching operationmay be either monostage stretching or multistage stretching. In the caseof biaxial stretching, it may be either simultaneous stretching orsequential stretching, but sequential stretching is preferred. In thecase of polypropylene, the stretching temperature is not higher than themelting point of the polypropylene, and preferably a temperature rangingfrom the softening point of the polypropylene to the melting pointthereof. Concretely, the stretching temperature is preferably 100-155°C., and more preferably 120-150° C. A stretching temperature lower than100° C. is undesirable because polypropylene is softened insufficientlyand, therefore, is not stretched with stability. On the other hand, astretching temperature higher than 155° C. is also undesirable becausepolypropylene is not oriented through the stretching.

[0020] In the above-mentioned stretching step, voids (vacancies bypores) are formed within a film through occurrence of relative shiftbetween the starting resin and incompatible resin or inorganic fillerforming the film to such an extent that the film does not break. Thus, alight-scattering white film can be obtained.

[0021] Next, explained is the method for obtaining a white film byforming bubbles through foaming by use of a foaming agent.

[0022] Any known foam molding technique may be adopted. Concretely,preferred is a method in which a foamed film is obtained by use of afoaming agent decomposition method comprising kneading a thermallydecomposable foaming agent, which is capable of generating decomposedgas on heating, into a thermoplastic resin such as polyolefin in advanceto form a film and then heating the film to a temperature equal to orhigher than the decomposition temperature of the foaming agent. Examplesof the thermally decomposable foaming agent include azodicarbonamide,azobisformamide, dinitrosopentamethylenetetramine andtoluenesulfonylhydrazide. In the formation of a sheet using an extruder,preferred is a method in which a foamed film is obtained by use of a gasentraining technique in which gas or a physical foaming agentvaporizable on heating is supplied under pressure through a pressurefeeding port formed in the extruder at its middle portion. Examples ofthe physical foaming agent include inorganic gases such as air,nitrogen, water and carbon dioxide gas; and organic foaming agents suchas flon, butane, pentane and hexane.

[0023] The expansion ratio is preferably 1.01-10 and more preferably1.1-3. One example of the method for obtaining a crosslinked foamed filmis a method in which melt kneading is conducted under a temperaturelower than the decomposition temperature of the thermally decomposablefoaming agent to afford a non-crosslinked and slightly-foamed film, acrosslinking aid is, if necessary, added, then the film is crosslinkedby, for example, exposure to an ionizing radiation, and then the film isheated at a temperature equal to or higher than the decompositiontemperature of the foaming agent to form the crosslinked foamed film.

[0024] In the white film obtained in such a way, because irradiation ofthe film with light results in irregular reflection of the light byvoids or bubbles, the surface of the film is whitened and, therefore,the film has a reduced light transmission. The light transmission of thewhite film must be not more than 50%, and is preferably not more than40%. The light transmission is measured by the ASTM-D1003 method.Regarding the whiteness of the white film as a measurement obtained bythe ASTM-E313 method, the film preferably has a whiteness of 70 or moreand more preferably a whiteness of 80 or more.

[0025] The thickness of the white film is preferably 10-100 μm and morepreferably 30-60 μm. In the present invention, the white film may beused in the form of a film consisting thereof as a light-shieldingmaterial. The white film may also be used in the form of tape-likelight-shielding material obtained by slitting the film into a narrowwidth of about 5-50 mm. Alternatively, a flat yarn obtained by slittingthe white film into a narrow width of about 2-20 mm may be used as alight-shielding material.

[0026] The following is a description on a reinforcement forconstituting the light-shielding material of the present invention incombination with the light-shielding material made of the white filmdescribed above. The reinforcement in the present invention is amaterial which is made of a textile fabric or a nonwoven fabric andwhich imparts physical strength to the entire light-shielding sheet.

[0027] The raw material of the textile fabric used for the reinforcementin the present invention is not restricted particularly and variouskinds of fibrous materials are available. For example, natural fiberssuch as cotton and hemp; regenerated fibers such as rayon; and syntheticfibers such as polyolefin, polyester, polyamide, acrylic fiber, vinylon,polyvinyl chloride and carbon fiber can be employed.

[0028] Of these materials, thermoplastic resins are preferable becausethey can be attached by thermal welding to other materials such as thewhite film. Examples of suitable thermoplastic resins include polyolefinsuch as polyethylene and polypropylene; polyester such as polyethyleneterephthalate and polybutylene terephthalate; and polyamide such asnylon 6 and nylon 6,6. Above all, polyolefin is suitably employed.Specific examples of the polyolefin include polyethylene resins such ashigh density polyethylene, medium density polyethylene, linear lowdensity polyethylene, branched low density polyethylene, ultra lowdensity polyethylene, linear low density polyethylene produced by use ofa metallocene catalyst, ethylene-vinyl acetate copolymer, ethylene-alkyl(meth)acrylate copolymer; and polypropylene resins such aspolypropylene, ethylene-propylene block copolymer and ethylene-propylenerandom copolymer. Of such polyolefins, crystalline polyolefin ispreferred. The crystalline polyolefin is suitable as a reinforcement fora light and tough light-shielding sheet because it can be improved instrength easily through a drawing operation and it is of light weight.The crystalline polyolefin, e.g. polyethylene and polypropylene, maycontain a small amount of copolymerized component unless thecrystallinity is seriously affected. Specific examples thereof includehigh density polyethylene, medium density polyethylene, linear lowdensity polyethylene, polypropylene and ethylene-propylene blockcopolymer. Above all, polypropylene is particularly preferable. Thesemay be used either alone or in combination. The melt flow rate(hereinafter abbreviated as MFR) of the polyolefin is normally withinthe range of 0.1-20 g/10 minutes, and preferably within the range of0.2-10 g/10 minutes.

[0029] It is preferable that the yarn used as a starting material be adrawn yarn because the resulting reinforcement has an improved strength.The single yarn size of the drawn yarn falls within the range of 50-500decitex (hereinafter, abbreviated as dt), and preferably 70-300 dt. Asingle yarn size less than 50 dt results in an insufficient reinforcingeffect; whereas a single yarn size over 500 dt is also undesirablebecause it leads to increased rigidity, which results in loss offlexibility and worsening of handlability.

[0030] As the form of the drawn yarn, any form may be adopted such as aflat yarn obtained by slitting and stretching a film, a split yarnobtained by splitting a flat yarn, a monofilament obtained by stretchinga filament extruded through a circular or profile nozzle, and amultifilament obtained by collecting small-size filaments. Of theseyarns, a flat yarn is preferred in view of formability and smoothness.In particular, it is preferable to use a smooth flat yarn as a startingmaterial because the reinforcement in the present invention is laminatedto or woven with a white film. The drawn yarn may be any type ofconjugate yarn such as a sheath-core type monofilament made of ahigh-melting component as the core and a low-melting component as thesheath and a parallel type flat yarn composed of a high-meltingcomponent as an intermediate layer and a low-melting component laminatedon both sides of the intermediate layer.

[0031] A textile fabric is formed by use of the thus-obtained yarn,particularly a drawn yarn made of polyolefin. The resulting fabric isused as a reinforcement. In particular, it is preferable that a wovenfabric obtained by weaving a polyolefin flat yarn as warps and wefts beused as a reinforcement. In this situation, a flat yarn made of a whitefilm, which is a light-shielding material, may be woven together in thewoven fabric.

[0032] The weave of the textile fabric is not particularly restricted.Examples of that of woven fabric include plain weave, twill weave, mockleno weave, gauze weave and leno weave. Examples of that of knittedfabric include raschel stitch, tricot stitch and milanese stitch. Theweight per unit area of the textile fabric is within the range of 50-300g/m², and preferably 70-150 g/m².

[0033] The nonwoven fabric used for the reinforcement in the presentinvention is not limited particularly. Nonwoven fabrics produced byvarious methods maybe employed. For example, either of meltblowing inwhich fibrous materials melt extruded using thermoplastic resin,especially polyolefin, are stacked randomly so that the molten fibrousmaterials are melt bonded with each other to form a sheet andspunbonding in which cooled fibrous materials are melt bonded with eachother on hot emboss rolls to form a sheet may be employed. Moreover, asheet-like material made of a laminated nonwoven fabric formed byarranging drawn yarns of thermoplastic resin longitudinally andlatitudinally at predetermined intervals, stacking and bonding them atintersections may also be used.

[0034] The textile fabric or the nonwoven fabric may be used alone as areinforcement. A laminate sheet obtained by laminating a laminationlayer made of thermoplastic resin on at least one side of the textile ornonwoven fabric may be used as a base fabric for the reinforcement.Examples of the thermoplastic resin used for the lamination layerinclude polyolefin, polyester such as polyethylene terephthalate andpolybutylene terephthalate, and polyamide such as nylon 6 and nylon 6,6.Polyolefin is suitably employed. Examples of the polyolefin used for thepolyolefin lamination layer include polyethylene resins such as highdensity polyethylene, medium density polyethylene, branched low densitypolyethylene, linear low density polyethylene, ethylene-α-olefincopolymer produced by use of a metallocene catalyst, ethylene-vinylacetate copolymer, ethylene-acrylic ester copolymer; and polypropyleneresins such as polypropylene homopolymer, ethylene-propylene blockcopolymer and ethylene-propylene random copolymer. These may be usedalone or in combination. The MFR of the polyolefin is normally withinthe range of 1-50 g/10 minutes, and preferably within the range of 5-30g/10 minutes.

[0035] As the method for forming the lamination layer in the textilefabric or the nonwoven fabric, known methods such as extrusionlamination, dry lamination and sandwich lamination maybe employed. Theextrusion lamination and the dry lamination are techniques of coveringone side of the textile or nonwoven fabric with the lamination resinlayer. The sandwich lamination is a technique of laminating the textilefabric or the nonwoven fabric and the white film via an adhesive layermade of a lamination resin disposed between the fabrics. The thicknessof the lamination layer is preferably 20-100 μm, and more preferably30-100 μm.

[0036] The light-shielding sheet of the present invention is required tohave a high weatherability because it is often used outdoor and, inparticular, is exposed to the sunlight for a long term. It, therefore,is desirable to incorporate a weathering agent such as a lightstabilizer or a UV absorbent to the light-shielding material and thereinforcement in order to improve the weatherablity. A hindered aminelight stabilizer is suitably employed as the light stabilizer. Theweathering agent is employed by being contained in at least one of thewhite film, a substrate and the lamination layer. Preferably, the agentis employed by being contained in all of the white film, the substrateand the lamination layer.

[0037] When a protective film is laminated on the front surface of thewhite film, the weathering agent may be contained in the protectivefilm. This can effectively prevent the light-shielding material and thereinforcement from deterioration. In such situations, the white film isnot required to contain the weathering agent. As the material of theprotective film, resins the same as those employed for the white film orthe lamination resin described previously may be employed. “Frontsurface”used herein means the surface located in the side in which thesunlight comes; whereas the “back surface” means the surface opposite tothe front surface. As the method for laminating the protective layer onthe front surface of the white film, known methods such as extrusionlamination, dry lamination and hot press lamination may be adopted. Fromthe viewpoint of formability, extrusion lamination is preferred. Thethickness of the protective film is preferably 20-100 μm.

[0038] Examples of the UV absorbent include benzophenone type UVabsorbents such as 2,4-dihydroxybenzophenone,2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-octoxybenzophenone and5,5′-methylenebis(2-hydroxy-4-methoxy benzophenones); benzotriazole typeUV absorbents such as 2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole,2-(2-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole and2,2′-methylenebis(4-t-octyl-6-benzotriazol)phenol; benzoate type UVabsorbents such as resorcinol monobenzoate,2,4-di-t-butylphenyl-3′-5′-di-t-butyl-4′-hydroxybenzoate andhexadecyl-3-5-di-t-butyl-4-hydroxybenzoate. The incorporation proportionof the UV absorbent is preferably 0.05-5% by weight, and more preferably0.1-1% by weight.

[0039] Examples of the hindered amine light stabilizer includebis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, tetrakis(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate,bis(1,2,2,6,6-pentamethyl-4-piperidyl)-di(tridecyl)-1,2,3,4-butanetetracarboxylate,a polycondensate of 1-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4-piperidinolwith diethyl succinate, and a polycondensate of1,6-bis(2,2,6,6-tetramethyl-4-piperidylamino)hexane with2,4-dichloro-6-t-octylamino-1,3,5-triazine. The incorporation proportionof the light stabilizer is preferably 0.05-15% by weight, morepreferably 0.05-5% by weight, and still more preferably 0.1-1% byweight.

[0040] In order to improve the weather resistance continuously for along term, it is preferable to use, as the hindered amine lightstabilizer, a high molecular hindered amine light stabilizer having ahindered amine as a side chain obtained by copolymerizing ethylene and acyclic aminovinyl compound. One example of the ethylene-based copolymeris a copolymer of ethylene and a cyclic aminovinyl compound produced byhigh pressure radical copolymerization described in detail in JP-A4-80215. One example of concrete products of the high molecular hinderedamine light stabilizer is KOKANOX (commercial name; manufactured byMitsubishi Chemical Corp). The incorporation proportion of the highmolecular hindered amine light stabilizer is preferably 0.1-15% byweight, and more preferably 1-10% by weight.

[0041] In the present invention, the combined use of the weatheringagent and the antioxidant shown below is preferable because alight-shielding sheet superior in heat resistance and in long-termdurability is obtained. Such an antioxidant may be incorporated into anyof the light-shielding material, the reinforcement and the protectivefilm. Phenol antioxidants, amine antioxidants, sulfur-containingantioxidants, phosphorus-containing antioxidants, etc. are available asthe antioxidant. Hindered phenol antioxidants are preferable. Inparticular, a combination use of a hindered phenol antioxidant and aphosphorus-containing antioxidant is preferred because the heatresistance is improved synergistically.

[0042] As the hindered phenol antioxidant, suitably employed is acompound having a phenol skeleton which has a t-alkyl group, e.g.t-butyl and t-amyl, on the carbon atom at one of its ortho positionswith respect to the phenolic hydroxyl group and also has a bonding to anatom other than hydrogen or carbon such as sulfur, oxygen and nitrogen,at least one position selected from the remaining ortho position withrespect to the phenolic hydroxyl group, the para position with respectto the phenolic hydroxyl group and the meta position with respect to thephenolic hydroxyl group which is also the para position with respect tothe t-alkyl group.

[0043] Specific examples of the hindered phenol type compounds suitablefor use in the present invention include n-octadecyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, pentaerythrityltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], triethyleneglycol bis[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate],3,9-bis[2-{3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane,tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate,2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenylacrylate, 2,4-di-t-amyl-6-[1-(3,5-di-t-amyl-2-hydroxyphenyl)ethyl]phenylacrylate,2,4-di-t-butyl-6-[1-(3,5-di-t-butyl-2-hydroxyphenyl)ethyl]phenylacrylate, 2,2′-ethylidenebis(4,6-di-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-thiobis(3-methyl-6-t-butylphenol),4,4′-methylene-bis(2,6-di-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol), and2,6-di-t-butyl-4-methylphenol. Above all, preferably employed are, forexample, n-octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate andpentaerythrityl tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate].The incorporation amount of these hindered phenol antioxidants is withinthe range of 0.02-1.0 part by weight, and preferably 0.05-0.2 part byweight.

[0044] In the present invention, it is preferable to use a hinderedphenol antioxidant and a phosphorus-containing antioxidant incombination because the heat resistance is improved synergistically.Specifically, a combination of pentaerythrityltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as a hinderedphenol antioxidant: tris(2,4-di-t-butylphenyl)phosphite as aphosphorus-containing antioxidant in a ratio of 1:1 to 1:4, acombination of octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate:tris(2,4-di-t-butylphenyl) phosphate in a ratio of 1:1 to 1:4, and thelike are preferable. The combined incorporation proportion of theantioxidants is 0.03-1.2% by weight, and preferably 0.05-0.7% by weight.An incorporation amount of the antioxidants of less than 0.03% by weightwill result in an insufficient heat resistance, whereas an incorporationof antioxidants over an amount of 1.2% by weight is uneconomical becauseno further improvement in heat resistance will be obtained.

[0045] In the present invention, combined use of a weathering agent andan antioxidant is particularly preferable. The combined use results in aremarkably improved heat resistance due to a synergistic effect. Themethod for incorporating the weathering agent and the antioxidant may bea method in which these substances are directly fed into an extrudertogether with a thermoplastic resin. However, a method in which a masterbatch containing a weathering agent and an antioxidant in highconcentrations is prepared and then the master batch is fedappropriately to an extruder is preferable in view of dispersibility.

[0046] To the white film and the thermoplastic resin for use in thepresent invention, additives commonly employed, such as a dispersingagent, a lubricant, an antistatic agent, pigment, inorganic filler, acrosslinking agent, a foaming agent and a nucleating agent, may beincorporated within an intended scope of the present invention.

[0047] In the present invention, when the white film is used in the formof a film consisting thereof as a light-shielding material, alight-shielding sheet 1 is formed, as shown in FIG. 1, by laminating abase fabric 2, as a reinforcement, which comprises a textile fabric or anonwoven fabric obtained by weaving a drawn yarn 7 as warps and wefts toa white film 4. The method for the lamination may be lamination using anadhesive. It, however, is preferable to laminate the white film 4 andthe base fabric 2 by forming a laminate layer 3 therebetween by sandwichlamination.

[0048] In the present invention, when tapes resulting from slitting thewhite film into a narrow width of about 5-50 mm are used as thelight-shielding material, alight-shielding sheet 1 is formed bylaminating the tapes 5 to the base fabric 2 which comprises a textilefabric or a nonwoven fabric obtained by weaving a drawn yarn 7 as warpsand wefts while spacing the tapes apart from one another approximatelyin parallel as illustrated in FIG. 2. The method for the lamination maybe lamination using an adhesive. It, however, is preferable to laminatethe tapes 5 formed from a white film and the base fabric 2 by forming alaminate layer therebetween by sandwich lamination.

[0049] In the present invention, when a flat yarn is formed by slittingthe white film into a narrow width of about 2-20 mm and then is used asthe light-shielding material, a light-shielding sheet 1 is formed bycombined weaving by using flat yarns 6 made of the white film as somewarps and/or wefts and also using drawn yarns 7, preferablymonofilaments or flat yarns, as other warps and/or wefts as illustratedin FIG. 3.

[0050] The light-shielding sheet obtained in the manner described aboveis suitably used for various applications such as light-shieldingmaterials for agricultural or horticultural use, such as houses andtunnels, light-shielding materials for livestock use, such as chickenhouses, pig houses and cow houses, light-shielding materials for works,such as protective sheets, and sheets for temporary tents for athleticmeetings, entertainments and the like. Of these applications, thelight-shielding sheet is most suitably used for light-shieldingmaterials for houses for agricultural or horticultural use orlight-shielding materials for tunnels for agricultural or horticulturaluse in view of light-shielding property, handlability, light weight anddurability.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051]FIG. 1 is a schematic sectional view of the light-shielding sheetaccording to Example 1 of the present invention.

[0052]FIG. 2 is a plan view of the light-shielding sheet according toExample 2 of the present invention.

[0053]FIG. 3 is a plan view of the light-shielding sheet according toExample 3 of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION EXAMPLE 1

[0054] In order to form a white film 4, 6% by weight of polybutyleneterephthalate (melting point: 223° C.) and 2% by weight of titaniumoxide were blended to polypropylene (MFR=2.0 g/10 minutes, density=0.90g/cm³) and the mixture was melt extruded into a sheet form by anextruder to form a 1.0 mm thick sheet. This sheet was 5-time stretchedby a roll stretcher in the longitudinal direction at a roll temperatureof 135° C. and then was 5-time stretched by a tenter stretcher in thetransverse direction to form a 40 μm-thick white film 4. This white film4 had a whiteness of 88 as measured by the ASTM-E313 method and a lighttransmission of 32% as measured by the ASTM-D1003 method.

[0055] On the other hand, a film was produced from polypropylene(MFR=2.0 g/10 minutes, density=0.90 g/cm³) by inflation method using anextruder. This film was slit into a width of 18 mm and then stretchedwith a drawing ratio of 6 times by hot rolling to yield a flat yarn 7with a size of 1000 dt. Using this flat yarn 7, a base fabric 2 made ofa plain woven fabric was formed with a longitudinal and transverseweaving density of 10 yarns×10 yarns/2.54 cm.

[0056] A 30 μm-thick lamination layer 3 was formed by laminating a lowdensity polyethylene (MFR=8.0 g/10 minutes, density=0.918 g/cm³) betweenthe white film 4 and the base fabric 2 by sandwich lamination. Thelaminate was fabricated into a light-shielding sheet 1. Thus, alight-shielding material for agricultural use and horticultural use wasformed (see FIG. 1). This light-shielding material for agricultural useand horticultural use was superior in light-shielding property,flexibility, handlability and durability and was employed suitably as acovering material for a fruit house.

EXAMPLE 2

[0057] A light-shielding material for agricultural use and horticulturaluse was obtained by forming a light-shielding sheet 1 in the same manneras Example 1 except that a 50 μm-thick white film having an expansionratio of 1.1 was obtained by blending 1% by weight of azodicarbonamideto a low density polyethylene (MFR=1.0 g/10 minutes, density=0.922g/cm³), melt extruding the mixture into a sheet form by an extruder, andfoaming the extrudate on heating. This white film 4 had a whiteness of85 as measured by the ASTM-E313 method and a light transmission of 38%as measured by the ASTM-D1003 method. This light-shielding material foragricultural use and horticultural use was superior in light-shieldingproperty, flexibility, handlability and durability and was employedsuitably as a covering material for a vegetable cultivation house.

EXAMPLE 3

[0058] The white film obtained in Example 1 was cut into 20 mm-widetapes 5, which were placed in parallel with a space of 10 mm on the basefabric obtained in Example 1. Lamination was conducted by sandwichlamination in the same manner as Example 1 to form a light-shieldingsheet 1. Thus, a light-shielding material for agricultural use andhorticultural use was formed (see FIG. 2). This light-shielding materialfor agricultural use and horticultural use was superior inlight-shielding property, flexibility and handlability and was employedsuitably as a covering material for an ornamental plant cultivationhouse.

EXAMPLE 4

[0059] A 4 mm-wide flat yarn 6 was prepared using the white filmobtained in Example 1. Using this flat yarn 6 as warps and thepolypropylene flat yarn 7 formed in Example 1 as wefts, a plain wovenfabric was formed with a longitudinal and transverse weaving density of10 yarns×10 yarns/2.54 cm. The plain woven fabric was fabricated into alight-shielding sheet 1. Thus, a light-shielding material foragricultural use and horticultural use was formed (see FIG. 3). Thislight-shielding material for agricultural use and horticultural use wassuperior in light-shielding property, flexibility, handlability anddurability and was employed suitably as a covering material for avegetable tunnel.

EXAMPLE 5

[0060] A white film was produced in the same manner as Example 1 exceptblending no titanium oxide in Example 1. As a result, the white film 4had a whiteness of 86 as measured by the ASTM-E313 method and a lighttransmission of 35% as measured by the ASTM-D1003 method. There was nogreat difference between the light transmission of this Example and thelight transmission (32%) of the white film produced in Example 1. Thissuggests that the contribution to the decrease in light transmissiondoes not result from the effect of titanium oxide as pigment but fromgeneration of voids within the film.

EXAMPLE 6

[0061] Operations were conducted in the same manner as Example 1 exceptthat in order to form a white film 4, a base fabric 2 and a laminationlayer 3, 3% by weight of KOKANOX (commercial name; manufactured byMitsubishi Chemical Corp.) as high molecular hindered amine lightstabilizer was blended to the starting resin of each of the white film,the base fabric and the lamination layer. A light-shielding sheet 1obtained through lamination demonstrated a change in color (determinedin accordance with JIS L 0804) on 5th rank after a weathering test (aweatherometer irradiation for 400 hours (corresponding to one year)using a xenon lamp under conditions: a temperature of 63° C. and ahumidity of 55%). Thus, the light-shielding sheet had a goodweatherability. The light-shielding sheet of Example 1 to which neitherweathering agent nor antioxidant had been added was subjected to a testthe same as that described above. The sheet demonstrated an apparentchange in color.

EXAMPLE 7

[0062] Operations were conducted in the same manner as Example 1 exceptthat a 30 μm-thick protective film of a polypropylene (MFR=2.0 g/10minutes, density=0.90 g/cm³) was extrusion laminated on a white film 4and that in the formation of the protective film, a base fabric 2 and alamination layer 3, 3% by weight of KOKANOX (commercial name;manufactured by Mitsubishi Chemical Corp.) as high molecular hinderedamine light stabilizer was blended to the starting resin of each of thewhite film, the base fabric and the lamination layer. A light-shieldingsheet 1 obtained through lamination demonstrated a change in color(determined in accordance with JIS L 0804) on 5th rank after aweathering test (a weatherometer irradiation for 400 hours(corresponding to one year) using a xenon lamp under conditions: atemperature of 63° C. and a humidity of 55%). Thus, the light-shieldingsheet had a good weatherability.

EXAMPLE 8

[0063] Operations were conducted in the same manner as Example 1 exceptthat in order to form a white film 4, a base fabric 2 and a laminationlayer 3, 3% by weight of KOKANOX (commercial name; manufactured byMitsubishi Chemical Corp.) as high molecular hindered amine lightstabilizer and 0.15% by weight of Irganox B215 (manufactured by CibaSpecialty Chemicals, a 2/1 mixture of pentaerythrityltrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]/tris(2,4-di-t-butylphenyl)phosphite as antioxidant were blended to the starting resin of each ofthe white film, the base fabric and the lamination layer. Thelight-shielding sheet 1 obtained through lamination was subjected to a98° C. gear oven test. The sheet demonstrated a residual tensilestrength of 99.8% after 119 days (corresponding to 10 years at normaltemperature) and, therefore, had enough durability. The light-shieldingsheet of Example 1 to which neither weathering agent nor antioxidant hadbeen added was subjected to a test the same as that described above. Thesheet demonstrated a residual tensile strength of 25.3%.

INDUSTRIAL APPLICABILITY

[0064] The white film used in the light-shielding sheet of the presentinvention can scatter the sunlight and can have a light transmission ofnot more than 50% because the film has voids or bubbles therein. Acombination of a light-shielding material made of this white film and areinforcement can provide a light-shielding sheet which has a lightweight and a high strength, which is superior in durability andproductivity, and which inhibits the temperature increase due toabsorption of heat from sunlight. The sheet, therefore, is employedsuitably, for example, as a light-shielding material of agricultural orhorticultural houses.

1. A light-shielding sheet comprising a light-shielding material made ofa white film having a light transmission of not more than 50% and areinforcement made of a textile fabric or a nonwoven fabric.
 2. Thelight-shielding sheet according to claim 1, wherein the white film is afilm having therein voids formed through stretching the film.
 3. Thelight-shielding sheet according to claim 1, wherein the white film is afilm having therein bubbles formed through foaming by use of a foamingagent.
 4. The light-shielding sheet according to claim 1, wherein atleast one of the light-shielding material and the reinforcement containsa weathering agent.
 5. The light-shielding sheet according to claim 1,wherein at least one of the light-shielding material and thereinforcement contains a weathering agent and an antioxidant.
 6. Thelight-shielding sheet according to claim 1, wherein a protective film islaminated on a surface of the light-shielding material and theprotective film contains a weathering agent.
 7. The light-shieldingsheet according to claim 1, wherein the reinforcement is a woven fabricobtained by weaving a drawn yarn made of polyolefin as warps and wefts.8. The light-shielding sheet according to claim 1, wherein thereinforcement is laminated on the back side of the white film.
 9. Thelight-shielding sheet according to claim 1, wherein tapes resulting fromslitting the white film into a narrow width are laminated on one side ofthe reinforcement while being spaced apart from one anotherapproximately in parallel.
 10. The light-shielding sheet according toclaim 1, wherein the light-shielding sheet comprises a woven fabricobtained by weaving a drawn yarn with a flat yarn as warps and/or wefts,the flat yarn resulting from slitting the white film into a narrowwidth.